Loud speaker and method of propagating sound



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Patented Jan. 15, 1 935 UNITED STATES LOUD srasxna AND ma'rnon or raora- GATING sormn Harry F. Olson, Collingswood, N. L, alaignor to Radio Corporation of America, a corporation of Delaware Application February 17, 1934, Serial No. 711,681

7 Claims. (01. 181-31) My invention relates to the art of propagating sound and in particular to means and a method for so converting electrical vibrations into sound waves as to control the low frequency response a of a loud speaker.

Devices of one sort, or another have heretofore been used for the purpose of giving proper emphasis to the sound waves within different frequency bands. Among the various systems that have been resorted to are those which employ a plurality of diaphragms suitably mounted in a single cabinet. In some cases it has been proposed to actuate each of these diaphragms from its own driving mechanism. In other cases it has been proposed to utilize additional diaphragms which are vibrated only by their own resonant characteristics. An example of the last mentioned type of construction was disclosed by W. D. La Rue in his Patent No. 1,787,946 dated January 6, 1931. La Rue has a loud speaker comprising a cabinet with cone diaphragms mounted in openings of the cabinet structure. One of these cones is directly driven by an electromagnetic device; the other cone vibrates freely in response to sound vibrations set up by the driven cone. This disclosure, however, does not teach anything with regard to the problem of increasing the low frequency response of aloud speaker.

An object of the present invention is to provide a loud speaker and a method of operating the same whereby the low frequency response thereof may be increased.

Another object of my invention is to develop a dependable method for lowering the cutoff point of the low frequency response in a loud speaker.

A further object is to increase the faithfulness of reproduction of sound in accordance with the original production thereof.

The foregoing and other objects of my invention will be best understood by reference to the accompanying drawing in which Figure 1 represents, more or less conventionally, a cross-sectional view of two cone diaphragms mounted within a sound chamber.

Fig. 2 shows a modified arrangement of a plurality of symmetrically disposed diaphragms, one of which is provided with an electro-dynamic driving mechanism.

Fig. 3 shows, for illustrative purposes, a conventional electrical filter system having electrical equivalents which may be compared with the acoustical properties of my loud speaker system.

Fig. 4 is a diagram showing the relation between frequency and the phase differences between two cones operated according to my invention, and

Fig. 5 is anothter diagram showing therelation between frequency and velocity of vibration of the diaphragms in my loud speaker.

In the conventional cone type of loud speaker it is customary to mount the cone within an opening in the front of the cabinet and to leave the back of the cabinet open. When the back of the cabinet is closed there is ordinarily a back wave which is objectionable in that it militates against faithfulness of reproduction, and, furthermore, it impedes the normal vibratory action of a single diaphragm. Moreover, the closing up of the cabinet in the rear renders the cabinet objectionably resonant at certain frequencies. The reaction upon the cone itself is such as to reduce the amplitude of vibration due to the phase difference of the reflected vibrations with respect to the force exerted by the driving mechanism.

Notwithstanding the difficulties heretofore encountered, when the diaphragm is mounted in an opening in a closed chamber I have found that the back wave may be substantially eliminated if one or more additional diaphragms are mounted within other openings in the sound chamber. I have found further that two cones operating in combination with the acoustical capacitance of a closed chamber may be so designed as to lower the cut-off frequency of the sound response. It will readily be seen, therefore, that in carrying out my invention it is posisble to arrive at a fundamental basis for constructing a loud speaker having the desired characteristics for reproducing sound, especially so as to give due emphasis to the low notes as well as those in the higher audible range.

Referring now to the drawing in detail, I show a cabinet having a sound chamber 1 enclosed by a suitable box 2 having openings 3 and 4 therein. Within the opening 3 is mounted an electrodynamically driven cone 5. The driving mechanism 6 for this diaphragm is conventionally shown. Mounted within the opening 4 is a second diaphragm 7 which has no driving mechanism of its own.

As a modification of the structure shown in Fig. 1, I show also in Fig. 2 an arrangement of three diaphragms symmetrically disposed. The diaphragm 5 is electro-dynamically driven as by the driving mechanism 6. The diaphragms 7a are mounted within additional openings. This arrangement has certain advantages from the standpoint of appearance. Other modifications might include the mounting of additional diaphragms (not necessarily cones) in the floor of the cabinet.

The characteristics of my loud speaker and its method of operation may best 'be' illustrated by reference to an equivalent electrical filter network such as is shown in Fig. 3. This diagram shows a conventional low pass filter such as would be constituted by two diaphragms, each having the same inertance (m1=mz) and coupled by an acoustic capacitance C. For such a filter it may easily be shown that the cut-off frequency f is given by the equation:

in other walls or an m =10 grams,

and further, that the area of each diaphragm is 320 sq. cm. Then Substituting in the formula for f:

Then

V V FT .oo1s (34,100) from which the value of V is found to be 230,000 cubic centimeters.

In Fig. 3 the relation between the inertances m1 and m2 and the capacitance C with respect to the radiation resistance n of the driven cone and the radiation resistance 12 of the second cone will be clearly understood, using the electrical equivalents thereof by way of illustration. The actuating power for propogating sound in the first place is represented at P in the diagram. As is well understood in the functioning of a filter system, a phase displacement takes place as between the two sides of the network. In like manner, my two cones 5 and '7 will respond with a certain phase displacement in respect to one another, depending upon the frequency of the sound vibrations. Above the cut-ofi frequency there is substantially a, phase difference of This means that as cone 5 moves out, cone 7 or cones 7a will move out. The velocity of the cone 7 or cones 7a will, however, be steadily decreased due to the filter action of the cone inertances in relation to the acoustic capacitance C of the sound chamber 1 for higher frequencies. This behaviour, however, is not objectionable because'it only helps to bring out the characteristics of my invention for discriminating in favor of the low frequencies. Thus, when cone 5 is vibrated in the low range, the phase displacement between the two cones 7 and 5 is reduced, thereby bringing their response more and more into unison. Thus, for the de sired low frequencies the velocity and hence the amplitude of vibration of the auxiliary cone 7 01' the cones 7a may be increased. This result can I also be obtained without any objectionable resonance such as would otherwise be caused by an enclosed sound chamber 1 if there were no auxiliary diaphragm mounted therewithin. The auxiliary cone, therefore, operates to eliminate the resonant point for a closed sound chamber.

The phase relations between the driven cone and the auxiliary cone are shown in Fig. 4. The

. illustrative.

The diagram of Fig. 5 is also illustrative of the characteristics of one embodiment of my invention. The broken line curve 9 represents the response of a single cone of conventional type. The full line curve 10, however, represents the combined response of two diaphragms actuated in accordance with my invention. It will be noted that a much more uniform and faithful response is obtained by the two diaphragms in combination, although at higher frequencies the auxiliary diaphragm adds very little to the performance due to the fact that it becomes inactive due to its large inertive reactance when the phase angle approaches The objects of my invention will, therefore, be seen to be accomplished in that the low frequency response is increased and at the same time the reflected wave occasioned when mounting a single cone within a closed air chamber has been eliminated.

It will be understood that the embodiments of my invention herein shown and described are illustrative of various forms in which loud speakers may be constructed. The spirit and scope of the invention, therefore, is to be understood as limited only as required in view of the prior art and as defined by the claims.

I claim as my invention:

1. In the propagation of sound from a loud speaker having a plurality of diaphragms, the method of controlling the low frequencycut-ofi point which consists in causing one only of said diaphragms to be actively associated with a source of electro-magnetic energization, projecting air vibrations forwardly and rearwardly of said energized diaphragm, impounding the rearwardly projected air vibrations, and causing the impounded air vibrations to react upon a second one of said diaphragms so as to set up vibrations thereof the phase relation of which with respect to the first said diaphragm is a function of the audiofrequency, thus tending to vibrate the two said diaphragms substantially in unison over a predetermined low frequency band.

2. In the propagation of sound from a loud speaker having a plurality of diaphragms, the method of controlling the low frequency cut-ofi' point which consists in causing one only of said diaphragms to be directly actuated from a source of electro-dynamic energization, projecting sound vibrations forwardly and rearwardly of said directly actuated diaphragm, impounding the rearwardly projected sound vibrations, causing the attenuation characteristics of the impounding region to discriminate between sound vibrations of difierent frequencies and causing the impounded sound vibrations within a given low frequency band to react upon said additional diaphragms to vibrate the same substantially in resonance with the first said diaphragm, but with diminishing amplitude as the limits of. said frequency band are exceeded.

3. In the propagation of sound from a loud speaker having a plurality of diaphragms, the method of lowering the low frequency cut-off point which consists in translating electrical vibrations at audiofrequencies into vibrations of one of said diaphragms, causing that diaphragm to generate air vibrations on both sides thereof, freely projecting the air vibrations in front of and impounding the air vibrations to the rear of said diaphragm, causing the impounded vibrations to be impressed upon a second of said diaphragms and causing the last mentioned diaphragm to respond to the impounded vibrations at various degrees of phase displacement for different audiofrequencies,

' 4. In a loud speaker, a sound propagating device having at least two diaphragms, an electrodynamic driver element operativelyv connected with one of said diaphragms, and a closed air chamber walled in part by said diaphragms, the combination being such that within a limited frequency band at least two of said diaphragms are vibrated in unison while beyond said frequency band the electro-dynamically driven diaphragm vibrates practically alone.

5. In a loud speaker, means for translating electrical energy into sound vibrations, said means comprising an electro-dynamically driven diaphragm, a second diaphragm, and means including an air chamber walled in part by both said diaphragms for communicating sound waves within a certain audiofrequency range from the first to the second of said diaphragms, and for acoustically uncoupling said diaphragms, one from another, when one is vibrated at frequencies above said range.

6. In a device for propagating sound, a plurality of diaphragms, electro-dynamic means for actuating one of said diaphragms, and means for impounding the air behind said diaphragms, said means offering definite attenuation characteristics in respect to sound vibrations of different frequencies propagated within its confines, and the inertance of said diaphragms being characterized in relation to the attenuation values of said impounding means whereby the additional diaphragms are caused to vibrate substantially in unison with the first of said diaphragms within a given low frequency band, but with diminishing amplitude in response to frequencies outside said band.

7. In a device for propagating sound, an orificed sound chamber, a plurality of diaphragms each mounted within a different orifice of said sound chamber, electric controlling means operatively connected with one of said diaphragms, and means including the effective inertance characteristics of said diaphragms, and the acoustic capacitance characteristic of said sound chamber for vibrating all of said diaphragms substantially in unison within a given low frequency range.

HARRY F. OLSON. 

